This invention relates to a focusing electrode for an inline electron gun structure for color cathode ray tubes (CCRT's), in which the apertures are tapered and overlapping; and more particularly relates to such electrode in which the small openings of the apertures are enlarged for electron beam spot-shaping; and also relates to gun structures incorporating such electrode.
Reducing the diameter of the necks of CCRT's can lead to cost savings for the television set maker and user in enabling a corresponding reduction in the size of the beam deflection yokes, leading to cost savings in both material and power consumption. However, reducing neck diameter while maintaining or even increasing display screen area severely taxes the performance limits of the electron gun.
In the conventional in-line electron gun design, an electron optical system is formed by applying critically determined voltages to each of a series of spatially positioned apertured electrodes. Each electrode has at least one planar apertured surface oriented normal to the tube's long or Z axis, and containing three side-by-side or "in-line" circular straight-through apertures. The apertures of adjacent electrodes are aligned to allow passage of the three (red, blue and green) electron beams through the gun.
Most such guns are based on a bipotential lens design, in which focusing is achieved in a lensing field provided by two or more electrodes divided into a low voltage portion and a high voltage portion, typically a low voltage focusing electrode (G.sub.3) and a high voltage accelerating electrode (G.sub.4). The lensing field is formed in the region of beam acceleration, i.e., inside the forward portion of the focusing electrode, in the gap between the forward aperture plane of the focusing electrode and the rearward aperture plane of the accelerating electrode, and inside the rearward portion of the accelerating electrode.
As the gun is made smaller to fit into the so-called "minineck" tube, the apertures are also made smaller and as is well known, the focusing or lensing abberrations of the focusing and accelerating electrode apertures are increased, thus degrading the quality of the resultant picture on the display screen.
Various design approaches have been taken to attempt to increase the effective aperture sizes of these lensing electrodes. For example, U.S. Pat. No. 4,275,332, and U.S. patent application Ser. No. 303,751, filed Sept. 21, 1981, describe overlapping lens structures. U.S. patent application Ser. No. 487,347, filed Apr. 21, 1983, describes a lens structure with enlarged apertures surrounded by a raised rim. U.S. patent application Ser. No. 463,791, filed Feb. 4, 1983, describes a "conical field focus" or CFF lens arrangement. Each of these designs is intended to increase effective aperture size in the main lensing electrodes and thus to maintain or even improve gun performance in the new "mini-neck" tubes.
In the CFF arrangement, a large effective aperture size in the focusing and accelerating electrodes is provided by apertures having the shapes of truncated cones or hemispheres. That is, each aperture has a large opening in the aperture plane and a related small opening in the electrode interior. The large openings of both the focusing and accelerating electrodes thus face each other across the gap.
In a preferred CFF embodiment, the effective aperture size of both electrodes is further increased by enlarging the apertures until their large openings overlap. This overlapping eliminates portions of the sidewalls between adjacent apertures, leaving arcuate "saddles" bridging these apertures across the in-line plane.
These saddles create asymmetric lenses having larger diameters in the direction of the in-line plane than in the transverse direction. In the focusing electrode, such asymmetry tends to create beam spots at the screen with severe horizontal elongation. Thus, for optimum performance of the overlapping CFF lens arrangement, the asymmetry in the focusing electrode must be fully compensated, such as by an effectively identical or "balancing" asymmetry in the accelerating electrode.
Such an identical or "balancing" asymmetry in the accelerating electrode cannot be achieved simply by creating identical facing saddles in this electrode, because, due to the potential difference across the gap, the beams have a higher velocity in, and their parths are less affected by the accelerating electrode than the focusing electrode.
Thus, in practice, the accelerating electrode apertures in the CFF gun are even further enlarged to deepen the saddles sufficiently to create a compensating asymmetry for the asymmetry of the focusing electrode. See concurrently filed U.S. patent application, Ser. No. 680,713.
In some new gun designs now being considered, identical parts are used for both the focusing and accelerating electrodes to minimize astigmatism caused by non-circularity of the apertures. See co-pending U.S. patent application Ser. No. 516,028, filed July 22, 1983, and assigned to U.S. Philips Corp. Of course, the use of such identical parts in the CFF lensing arrangement prevents the opportunity for balancing of the horizontal asymmetry due to the saddles in the focusing electrode.
It is an object of the present invention to provide a focusing electrode with overlapping tapered apertures which has a vertical asymmetry sufficient to substantially compensate for the horizontal asymmetry of the saddles. Such an electrode is referred to herein as a "self-balancing" electrode.
It is a further object of the present invention to provide a modified bipotential lens electron gun structure incorporating a self-balancing focusing electrode, which modified structure will enable the use of identical parts for both the focusing and accelerating electrodes, without significant distortion of the beam spots at the screen.